GasBuddy.com calculated the fuel-cost-per-mile by taking the combined EPA fuel economy ratings for each vehicle and divided that by the national average for a gallon of unleaded gas.

By comparison, the Bugatti Veyron, a 16-cylinder supercar that sells for $2.25 million dollars, is the most expensive car to fuel, coming in at 36 cents per mile.

"I suppose you could argue that someone driving a 16-cylinder car like the Veyron isn't worried about spending 36 cents per mile to drive their car!" said Tom Kloza with GasBuddy.com. "But for most people, this is a good gauge of how fuel efficient their car is out on the road," he said.

GasBuddy.com says the cars that cost the most to fuel uptend to be the most expensive cars, including ultra luxury sports cars and bigger, heavier trucks and vans.

Highest fuel-cost-per-mile

Bugatti Veyron (35.9 cents/mile)

Ford E350 Wagon (32.6 cents/mile)

Chevrolet 2500 Suburban (29.9 cents/mile)

GMC 2500 Yukon Xl (29.9 cents/mile)

Lamborghini Aventador (29.9 cents/mile)

GasBuddy.com also broke the list down by automaker and individual brands that carry the lowest cost to fuel up. Topping the list — Smart, maker of the Smart car.

Overall, most people spent less per mile on fuel in July of this year compared to July of 2012. That's a reflection of the moderate gas prices we've seen this summer.

What's a fair price to pay per mile?

According to GasBuddy.com, almost 500 vehicles cost less than 20 cents per mile to drive. What's a reasonable expectation? Kloza says anything under 19 or 20 cents per mile. "19 or 20 cents per mile is a fair expectation," says Kloza. "You can get a great car for that price and a lot lower without having to give up much."